Wire drawing is the method used to produce most wire products. By way of example, consider the fabrication process of multifilamentary composite superconducting wire—a technology of particular interest for the present invention. One common wire type used for superconducting electromagnets consists of Nb-47 wt % Ti filaments within a Cu matrix. As is known in the prior art, such composite wires are commonly produced by stacking an assemblage of Nb—Ti alloy rods into a copper tube or can, extruding the assemblage to rod, and drawing the rod into the wire used for magnet winding. Small defects or flaws can occasionally occur internally in the resulting wire, caused for example by foreign material particles inadvertently incorporated into the composite during billet assembly, or flaws formed during drawing due to inadequate bonding at interior surface interfaces, such as where the Nb—Ti rods interface with the copper matrix.
Although undesirable, flaws are a nearly unavoidable practical occurrence in the large-scale production of wire. This reality is in conflict with the requirement that a wire must be absolutely free of flaws to be functional in persistent-current superconducting magnets, for applications such as Magnetic Resonance Imaging (MRI). Thus, it is a critical quality assurance step for the manufacturing of superconducting wire that flaws are detected and removed from the final wire before it is used in a magnet.
A prior art technique for finding flaws, eddy current testing, is commonly used in wire manufacturing for flaw detection. However, given the heavily surface biased phenomenon nature of the eddy current signal, and the fact that the superconducting filaments in a wire are typically far from the wire surface, eddy current testing alone is often inadequate for detecting the presence of filament breaks in the wire interior. Thus, additional means of detecting or eliminating such flaws in the internal wire structure are desirable.
According to the theory of bending, as described by Dieter (Mechanical Metallurgy, G. E. Dieter, McGraw-Hill, Inc., 1986), the bending strain of a specific ‘fiber’ at a given location within the wire varies across the wire diameter. At the middle of the wire thickness is a neutral axis at which the strain on a wire fiber is zero. The strain at other locations within the wire thickness is proportional to the distance from this neutral axis, with the fibers on the outer wire surface being strained in tension, and the fibers on the inner surface being compressed. For the purposes of bending a wire so as to open (break) its flaws, it is the tensile strain that is important; compressive strains at a flaw (e.g. a crack) will not serve to open or magnify the flaw. This aspect of bending is germane to the present invention, i.e. the fact that the only wire section placed into tension is the outer surface of the wire in contact with the roller. The cumulative result of these actions is that it turns an internal flaw into a surface component, more easily detected by conventional eddy current testing and high-speed laser micrometer measurement.
Wire straighteners, also known as cast-killing rolls, have originally been commercially produced to remove or “kill” the cast of a wire. In this method, a set of small diameter rollers is placed within the wire movement path. The typical wire straightener roll set consists of three rollers arranged at the vertices of a triangle. The diameter of the rollers is small enough to cause the wire to experience significant bending strain as the rollers bend the wire. Traditionally this strain is used to remove cast from a wire. Although these wire straighteners could be used to gain some of the benefit of the present invention, prior art straightening rolls are not optimized for the purposes of the present invention.
While it is thus possible to use prior art straightening rolls to expose internal flaws of a multifilamentary wire, the drawbacks are as follows:
1) In prior art roller arrangements, the wire is in contact with the rollers over just a few degrees of the roller circumference, resulting in little penetration of bending strain into wire
2) Typical straightening rolls setups are in one plane, sometimes multiple planes, but never have the forward and reverse bends in each plane of operation. This is a critical improvement in the invention disclosed here.
3) By itself, wire straightening rolls may amplify a defect yet not break the wire, allowing a defect to pass into final product. It is more desirable for the bending process to actually break the wire, ensuring it is not allowed to pass into final product.